There are various methods of separating gases. One particular method is membrane separation which employs the permselectability of a membrane at various conditions to facilitate separation of one or more gases. Industrial applications demand separation membranes that are highly selective and have very good mechanical properties at high temperatures.
Organic membranes are typically employed to provide a gas phase separation medium which is very selective. Unfortunately, these selective membranes particularly the polysiloxane membranes do not possess good mechanical properties at higher temperatures.
On the other hand, inorganic membranes do possess excellent high temperature mechanical properties. These membranes are generally formed from various vitreous compositions such as porous glass particularly Vycor glass manufactured by Corning Glass Works. Unfortunately, these inorganic microporous membranes do not possess the requisite selectivity for many industrial applications.
Accordingly, there is a need to combine the benefits of an organic polysiloxane membrane with an inorganic membrane.
Unfortunately, the end use requirements demand that first the inorganic support must in fact be a microporous support. If the pores are too large the membrane itself will be too weak and will rupture. This compounds the problem that the organic portion must be evenly distributed throughout the inorganic member. For example, simply coating the surface of an inorganic support member with a polydimethylsiloxane film does not provide a good separation medium.
There have been attempts to provide a heterogeneous film formed by in situ polymerization of a polydimethylsiloxane on an inorganic support member. For example, Shroder et al, Physics of Thin Films Vol. 5, Academic Press, NY 87-140 (1969) discusses a process for the modification of the surface of an inorganic media with a synthetic polymer. The polymer or prepolymer is converted to a liquid state either by melting or dissolving. It is then coated on the outside surface. This is an ineffective separation medium.
Kaiser et al, Journal of Membrane Science 22, 257-268 (1985) discuss various methods of polymerization of polydimethlsiloxane on a glass scrim filter medium. These primarily discuss methods whereby the polydimethylsiloxane film is formed on an inorganic support positioned between two reactive liquid solutions, an aqueous catalyst solution and an aqueous solution of the monomer. When the monomer is in an aqueous solution it tends to react to form a prepolymer with the cyclic tetramer predominating in the resulting mixture. This does not effectively form a polymer within an inorganic member.
The Kaiser article also discloses a gas phase reaction. The inorganic support is soaked with catalyst solution and placed above a silane bath. Vapors from the bath react when contacting the catalyst solution. This forms a silicone coating on the exterior surface of the support. Because the support layer is soaked in the catalyst it is impossible to accurately control the thickness of the coating. It is further desirable to have the silicone material inside the porous material and simply not on the surface of the porous material.